This invention relates to control circuitry for electrical circuits, and more particularly to improved circuitry in a power supply for sensing the load current provided thereby and limiting the maximum value thereof.
Regulated power supplies are employed to convert an input line voltage to a relatively constant supply voltage of a different value for driving an external load. A prior art DC-to-DC switching converter is described in Universal Switching Regulator Subsystem, Fairchild .mu.A78S40, Preliminary Data Sheet, December 1977. Switching converters are also described in Switching Regulator Operation, Fairchild Progress, Journal of Semiconductor, pgs. 19-24, vol. 7 #2 March/April 1979. Prior Art AC-to-DC voltage converter type power supplies are illustrated in FIGS. 12 and 13 of Hewlett Packard Application Note #90A, DC Power Supply Handbook, copyright 1970.
Current limiting is important in any form of regulator and converter in order to prevent the series connected active element thereof dissipating excessive power under short circuit overload conditions. In an elementary prior-art circuit for sensing load current in a switched converter, the base-emitter junction of a control transistor is connected across a series current monitoring resistor, the collector of the control transistor being connected to the base of a switching transistor. When the voltage across the monitor resistor is approximately 0.7 volt for a prescribed load current, the control transistor conducts to turn off the switching transistor. In most instances, this means that the resistance of the monitor resistor must be small and a precise value for turning off the switching transistor for a prescribed load current. By way of example, the resistance of the monitor resistor is only 0.35 ohm for a load current of 2.0 amperes. This means that the monitor resistor must be a precision value factory select resistor, increasing the cost of and time to produce such a switching converter. In the constant current power supplies in FIGS. 12 and 13 of the Hewlett Packard handbook, supra, the feedback loop associated with a series current monitor resistor comprises a Zener diode that is connected across a reference voltage regulator circuit, one side of the diode being directly electrically connected to one side of the monitor resistor and to one input of a differential amplifier. The other side of the Zener is electrically connected through a fixed resistor and a variable resistor that are connected in series to the other side of the monitor resistor, the junction of the series connected resistors being connected to the other input of the differential amplifier that produces an output control voltage that is applied to the base of a series regulator transistor. This prior art current feedback loop requires a reference power supply, an additional complication that reduces efficiency, and a Zener diode to produce the voltage reference. This means that the voltage reference is temperature sensitive, as is the current control. In addition, the Zener's temperature characteristics cannot be readily matched to temperature characteristics of resistors there in order to make current control independent of operating temperature.
Further, one side of the input to the constant current comparison amplifier is connected to the output circuit. This means that the amplifier itself must have a separate power supply, since connecting this input and the amplifiers' positive bias supply terminal to the same line is impractical. An object of this invention is the provision of an improved current sensing circuit for a power supply.